During the sixteenth and seventeenth centuries, dozens of alliances asserting shared sovereignty formed in the Holy Roman Empire and the Low Countries. Many accounts of state formation struggle to explain these leagues, since they characterize state formation as a process of internal bureaucratization within individual states. This comparative study of alliances in the Holy Roman Empire and the Low Countries focuses on a formative time in European history, from the late fifteenth century until the immediate aftermath of the 1648 Peace of Westphalia, to demonstrate how the sharing of sovereignty through alliances influenced the evolution of the Empire, the Dutch Republic, and their various member states in fundamental ways. Alliances simultaneously supported and constrained central and territorial authorities, while their collaborative policy-making process empowered smaller states, helping to ensure their survival. By revealing how the interdependencies of alliance shaped states of all sizes in the Empire and the Low Countries, Christopher W. Close opens new perspectives on state formation with profound implications for understanding the development of states across Europe.